I'd like to edit programs for the 50g on my PC and then transfer to the calculator. While this is quite possible in text-mode of GNU emacs, a dedicated rpl-mode could facilitate things like entering the trigraphs etc. So before I delve into emacs lisp, I thought I'd see if anyone had already attempted this.

I've never seen anything, but I'd love an emacs RPL or RPN mode as well.

In fact, now that you bring it up, I don't know why I hadn't thought of it before. If you work on the RPL mode, I'll work on the RPN mode! It shouldn't be too difficult to get elisp to handle automatic line numbering, label detection, and GTO/XEQ line renumbering.